In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) is used. Specifically, the PS plate is exposed through a mask, for example, a lith film, and then subjected to development processing, for example, with an alkaline developer to remove the unnecessary image-recording layer corresponding to the non-image area by dissolving while leaving the image-recording layer corresponding to the image area, thereby obtaining the lithographic printing plate.
Due to the recent progress in the technical field, nowadays the lithographic printing plate can be obtained by a CTP (computer-to-plate) technology. Specifically, a lithographic printing plate precursor is directly subjected to scanning exposure using laser or laser diode without using a film and developed to obtain a lithographic printing plate.
With the progress described above, the issue on the lithographic printing plate precursor has transferred to improvements, for example, in image-forming property corresponding to the CTP technology, printing property or physical property. Also, with the increasing concern about global environment, as another issue on the lithographic printing plate precursor, an environmental problem on waste liquid discharged accompanying the wet treatment, for example, development processing comes to the front.
In response to the environmental problem, simplification of development or plate making or non-processing has been pursued. As one method of simple plate making, a method referred to as an “on-press development” is practiced. Specifically, according to the method after exposure of a lithographic printing plate precursor, the lithographic printing plate precursor is mounted as it is on a printing machine without conducting conventional development and removal of the unnecessary area of image-recording layer is performed at an early stage of printing step.
Also, as a method of simple development, a method referred to as a “gum development” is practiced wherein the removal of the unnecessary area of image-recording layer is performed using not a conventional high alkaline developer but a finisher or gum solution of near-neutral pH.
In the simplification of plate making operation as described above, a system using a lithographic printing plate precursor capable of being handled in a bright room or under a yellow lump and a light source is preferable from the standpoint of workability. Thus, as the light source, a semiconductor laser emitting an infrared ray having a wavelength of 760 to 1,200 or a solid laser, for example, YAG laser, is used. An UV laser is also used.
As the lithographic printing plate precursor capable of undergoing on-press development, for instance, a lithographic printing plate precursor having provided on a support, an image-recording layer (photosensitive layer) containing an infrared absorbing agent, a radical polymerization initiator and a polymerizable compound is described in JP-A-2002-287334 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). Also, as the lithographic printing plate precursor capable of undergoing gum development, for instance, a lithographic printing plate precursor wherein the development is conducted with a finisher or a gum solution having pH close to neutral is known as described in EP-A-1751625 or EP-A-1868036.
In such a lithographic printing plate precursor of a simple processing type, a support having a surface of high hydrophilicity is heretofore used in order to make possible development with a developer having low pH or dampening water (ordinarily nearly neutral) on a printing machine and as a result, the image area is apt to be removed from the support by dampening water during printing so that sufficient printing durability can not be obtained. On the contrary, when the surface of support renders hydrophobic, ink also adheres on the non-image area during printing to cause printing stain. Thus, it is extremely difficult to achieve a good compatibility between the printing durability and the stain resistance and further improvements are desired.
In view of the above problems, in JP-A-2005-125749 or JP-A-2006-239860, a lithographic printing plate precursor capable of forming, an image without conducting alkali development and comprising a hydrophilic support having thereon a photopolymerizable layer sensitive to laser, wherein a copolymer which contains (a1) a repeating unit having at least one ethylenically unsaturated bond and (a2) a repeating unit having at least one functional group capable of interacting with a surface of the support is contained in the photopolymerizable layer or other layer is proposed and it is described that a lithographic printing plate which is excellent in not only printing durability but also stain resistance is obtained.
Also, in JP-A-2008-213177, a lithographic printing plate precursor comprising a support having thereon a hydrophilic layer composed of a hydrophilic polymer which contains at least one of a reactive group capable of directly chemically bonding with a surface of the support and a reactive group capable of chemically bonding with a surface of the support through a crosslinked structure and a partial structure having a positive charge and a negative charge and which is chemically bonded to the surface of the support and an image-forming layer in this order is proposed and it is described that a lithographic printing plate which is excellent in hydrophilicity of the non-image area and its sustention and also excellent in adhesion property between the image area and the support is obtained.
Further, in JP-T-2008-510634 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application), a lithographic printing plate precursor comprising a support having thereon an undercoat layer of a copolymer containing (1) a phosphonic acid group and/or a phosphate group and (2) an acid group and/or a group having an ethylene glycol or polyethylene glycol side chain is proposed.
However, with respect to the lithographic printing plate precursor proposed in JP-A-2005-125749 or JP-A-2006-239860, the stain resistance of the lithographic printing plate prepared is insufficient and further a problem arises in that stain occurs when the lithographic printing plate precursor is used after the passage of time from the preparation of lithographic printing plate precursor, that is, the stain resistance after the passage of time is insufficient.
Also, with respect to the lithographic printing plate precursor proposed in JP-A-2008-213177, sufficient results in both the stain resistance without the passage of time and the stain resistance after the passage of time are not obtained. Further, the hydrophilic polymer described in JP-A-2008-213177 has restriction on the producing condition, for example, in that it is necessary to use a nonaqueous solvent at the polymerization reaction in order to introduce the reactive group capable of chemically bonding with a surface of the support through a crosslinked structure into the polymer and a large environmental load.
Further, the lithographic printing plate precursor proposed in JP-T-2008-510634 has a problem of insufficient stain resistance.